Early Posting

Accepted papers to appear in an upcoming issue

Optica Publishing Group posts prepublication articles as soon as they are accepted and cleared for production. See the FAQ for additional information.

Terahertz Time-Domain Ellipsometry: Tutorial

Zahra Mazaheri, can koral, Antonello Andreone, and Antigone Marino

DOI: 10.1364/JOSAA.463969 Received 17 May 2022; Accepted 28 Jun 2022; Posted 30 Jun 2022  View: PDF

Abstract: Ellipsometry is a powerful, reference-free, non-invasive technique for the characterisation of materials, extensively used in the optical regime to investigate the properties as well as to evaluate the surface roughness and thickness of thin films and multilayered systems. Due to the inherent non-coherent detection technique, data analysis in optical ellipsometry mainly relies on the use of Jones formalism and is strongly linked to the specific setup, in most cases resorting to a rotating element (analyzer or compensator) to record the ellipsometric parameters. Aim of this tutorial is to provide an overview of the emerging field of ellipsometry in the THz region, putting in evidence similarities and differences with respect to the classical optical counterpart. THz time domain ellipsometry is based on a coherent detection approach and provides therefore in a simple and direct way the measurement of the material response, with no need to use a rotating element in the characterisation system. After giving a brief description on the technology presently used to generate and detect THz radiation, we introduce the general features of an ellipsometric setup operating in this frequency range, including advantages and limitations, illustrate the main equations governing the extraction of the material properties, and discuss calibration and setup issues affecting measurement sensitivity and accuracy. To back up and validate the study, results of measurements carried out on some common solids and liquids are presented.

Orbital angular momentum spectrum of pin-like optical vortex beams in turbulent atmosphere

Jiaxuan Cao, Lu Han, Huijian Liang, Gaofeng Wu, and Xiaoyan Pang

DOI: 10.1364/JOSAA.464275 Received 20 May 2022; Accepted 28 Jun 2022; Posted 30 Jun 2022  View: PDF

Abstract: The analytical formula of the probability density of a single orbital angular momentum (OAM) mode for pin-like optical vortex beams (POVBs) in turbulent atmosphere is derived. Its OAM spectrum in the receiving plane is obtained by the numerical calculation. For comparing, that of the commonly Gaussian vortex beams is showed, too. Those results show that POVBs show good performance on resisting the crosstalk of the OAM mode induced by the turbulence in some cases, such as smaller radius of the receiving aperture, longer propagation distance, stronger turbulence. Our finding has application on free space optical (FSO) communication based on the OAM mode.

Superimposed Hermite-Gaussian correlated Schell-model beam with multiple off-axis vortices

Jun Qu, zhenglan zhou, Feng Xu, Yuan Zhou, shaohua Zhang, Yangsheng Yuan, shuai han, Zhengxian Zhou, and Baoli Yao

DOI: 10.1364/JOSAA.462129 Received 27 Apr 2022; Accepted 24 Jun 2022; Posted 28 Jun 2022  View: PDF

Abstract: We first introduce aclass of superimposed Hermite-Gaussian correlated Schell-modelwith multiple off-axis vortices beam, with the side lobe of the beam carrying one to four vortex singularities at the source plane. Subsequently, the variation laws of this beam after being focused by a thin lens are studied theoretically to obtain the optimal beam parameters. The numerical simulation resultsshow that the beam possesses unique multiplevortex structure, phase structure, and orbital angular momentum. Its intensity resembles a spiralstaircase rotatingaround the axes. The rotational symmetry property of the transverse energy flow along the z-axis was broken by the vortices. The hot spot position can be adjusted flexibly by changing off-axis distance of vortices. This study is of great significancefor non-destructive capture and manipulation of multiple particles or cells.

Fusion network based on dual attention mechanism and atrous spatial pyramid pooling for automatic segmentation in retinal vessel images

Bingtao Liang, Chen Tang, Min Xu, Tianbo Wu, and Zhenkun Lei

DOI: 10.1364/JOSAA.459912 Received 31 Mar 2022; Accepted 23 Jun 2022; Posted 28 Jun 2022  View: PDF

Abstract: Accurate segmentation of retinal blood vessels from retinal images is crucial to aid in the detection and diagnosis of many eye diseases. In this paper, a dual attention - atrous spatial pramid polling network (DAANet) is proposed for vessel segmentation. Firstly, we propose a dual attention module consisting of a Position attention module and a Channel attention module, which aims to adaptively recalibrate features to extract effective features. And full-scale skip connections are used in the encoder to provide multi-scale feature maps for the dual attention modules. Then, the atrous spatial pyramid pooling (ASPP) allows the network to capture features at multiple scales and combine high-level semantic information with low-level features through the encoder-decoder architecture. We qualitatively evaluate the model using five metrics: sensitivity, specificity, accuracy, AUC and F1 score on DRIVE, CHASED_B1 and STARE datasets. DAANet outperforms the work of 10 state-of-the-art predecessors in these three datasets. Furthermore, we apply the trained model to clinical retinal images. The model obtains gratifying accurate and detailed segmentation results, which demonstrates promising application prospect in medical practices.

Image Reconstruction in Non-Reciprocal Broken-Ray Tomography

Matthew Faulkner, John Schotland, Vadim Markel, and lucia Florescu

DOI: 10.1364/JOSAA.461150 Received 14 Apr 2022; Accepted 23 Jun 2022; Posted 23 Jun 2022  View: PDF

Abstract: Optical methods of biomedical tomographic imaging are of considerable interest due to their non-invasive nature and sensitivity to physiologically important markers. Similar to other imaging modalities, optical methods can be enhanced by utilising extrinsic contrast agents. Typically, these are fluorescent molecules, which can aggregate in regions of interest due to various mechanisms. In the current approaches to imaging, the intrinsic (related to the tissue) and extrinsic (related to the contrast agent) optical parameters are determined separately. This can result in errors, in particular, due to using simplified heuristic models for the spectral dependence of the optical parameters. Recently, we have developed the theory of non-reciprocal broken-ray tomography (NRBRT) for fluorescence imaging of weakly-scattering systems. NRBRT enables simultaneous reconstruction of the fluorophore concentration as well as of the intrinsic optical attenuation coefficient at both the excitation and the emission wavelengths. Importantly, no assumption about the spectral dependence of the tissue optical properties is made in NRBRT. In this study, we perform numerical validation of NRBRT under realistic conditions using the Monte Carlo method to generate forward data. We demonstrate that NRBRT can be used for tomographic imaging of samples of up to four scattering lengths in size. The effects of physical characteristics of the detectors such as the area and the acceptance angle are also investigated.

Identification and separation of chiral particles by focused circularly-polarized vortex beams

Yanan Zhang, Manman Li, Shaohui Yan, Yuan Zhou, Wenyu Gao, and Baoli Yao

DOI: 10.1364/JOSAA.462817 Received 02 May 2022; Accepted 22 Jun 2022; Posted 23 Jun 2022  View: PDF

Abstract: The identification and separation of chiral substances are of importance in biological, chemical and pharmaceutical industries. Here, we demonstrate that a focused circularly-polarized vortex beam can, in the focal plane, selectively trap and rotate chiral dipolar particles via radial and azimuthal optical forces. The handedness and topological charge of the incident beam have strong influences on identifying and separating behavior: left- and right-handed circular polarizations lead to opposite effects on the particle of trapping and rotating, while the sign of topological charge will change the rotation direction of the particle. Such effects are a direct result of the handedness and topological charge manifesting themselves in the directions of the spin angular momentum (SAM) and Poynting vector. The research provides an insight into the chiral light-matter interaction and may find potential application in identification and separation of chiral nanoparticles.

Optical trapping of Rayleigh particles based on the four-petal Gaussian vortex beams

Yuge Liang, Yu Su, Jinhong Li, and Chen Yang

DOI: 10.1364/JOSAA.463732 Received 12 May 2022; Accepted 22 Jun 2022; Posted 23 Jun 2022  View: PDF

Abstract: The intensity distribution of a four-petal Gaussian vortex beam through a focused optical system and the radiation force acting on a Rayleigh dielectric sphere is obtained based on the extended Huygens-Fresnel principle and the Rayleigh scattering theory. We mainly study the trapping of high and low refractive index Rayleigh particles by the four-petal Gaussian vortex beam and the effect of the topological charge m on the radiation force. The results show that the specific distribution of the optical field of the incident beam can be controlled by a reasonable choice of the topological charge m to achieve the simultaneous capture of multiple Rayleigh particles with different refractive indices at different positions of the focal plane position using a single beam.

Incompatibility of the "Generalized Laws of Reflection and Refraction" with the basic axioms of electromagnetic wave propagation

Markus Schake

DOI: 10.1364/JOSAA.460037 Received 01 Apr 2022; Accepted 21 Jun 2022; Posted 22 Jun 2022  View: PDF

Abstract: The widespread concept of "Generalized Laws of Reflection and Refraction" that is commonly applied to wave propagation through metasurfaces is demonstrated to be incompatible with the basic axioms of electromagnetic wave propagation. A diffraction-based explanation of the reported phenomena is provided that yields a solid theoretical foundation for the prediction of experimental results and that resolves many of the contradicting explanations found throughout the literature.

Improvement of MCS-based Turbulent Induced Scattering Model for UWOC Channel

Peng Yue, Dongling Xu, Yi Xiang, and Jingyi Liu

DOI: 10.1364/JOSAA.459753 Received 30 Mar 2022; Accepted 16 Jun 2022; Posted 17 Jun 2022  View: PDF

Abstract: The light propagating in an underwater wireless optical communications (UWOC) channel suffers absorption and scattering effects jointly caused by particles and turbulence. By using Monte-Carlo simulation (MCS), most of the researches involving UWOC channel modeling have sufficiently considered the attenuation caused by particles, while ignoring or erroneously considering the absorption and scattering effects induced by turbulence, which will result in an inaccurate estimation of channel characteristics. Motivated by this, we use MCS method to construct a completer and more reasonable channel model, which makes up for the deficiencies of previous studies and provides a general analysis framework for the absorption and scattering effects brought by the two factors of particles and turbulence. We further study the path loss, channel impulse response (CIR), and probability density function (PDF) of the light intensity under different communication scenarios. Results show that compared to the situation involving only particle effects, the addition of consideration of turbulence effects increases the path loss by more than 10 dB, reduces the CIR amplitude to one-fifth, and makes the light intensity PDF become more dispersed. Our research can provide certain theoretical guidance for UWOC system design and performance evaluation.

Numerical optical propagation using sinc approximation

Max Cubillos and Edwin Jimenez

DOI: 10.1364/JOSAA.461355 Received 15 Apr 2022; Accepted 16 Jun 2022; Posted 16 Jun 2022  View: PDF

Abstract: We present a numerical method based on sinc series approximations for computations of optical propagation diffraction integrals. The exact diffraction integrals, which are convolutions of a kernel and a known optical field, can be shown to preserve the bandwidth of the source optical field. This property has important accuracy implications and is, in general, not satisfied by numerical methods such as the well-known angular spectrum method (ASM). Additionally, FFT-based methods introduce artificial periodicity which can further deteriorate numerical results. Our proposed numerical approach is not only super-algebraically convergent but it also satisfies this important bandwidth preservation property. Moreover, the accuracy of the proposed method is independent of wavelength, propagation distance, and observation plane discretization; it depends only on the accuracy of the source field approximation. Based on a detailed error analysis, we derive parameter selection criteria to achieve a prescribed error tolerance. Numerical simulations of Gaussian beam and optical phased array propagation are presented to verify the high-order accuracy of the proposed algorithms. To facilitate the reproduction of numerical results, we provide a Matlab code that implements our numerical approach for the Fresnel diffraction integral. For comparison, we also present numerical results obtained with the well-known angular spectrum method.

Metrics of color-difference formula improvement

Robert Carter, Pedro Garcia, Manuel Melgosa, and Michael Brill

DOI: 10.1364/JOSAA.461542 Received 28 Apr 2022; Accepted 15 Jun 2022; Posted 16 Jun 2022  View: PDF

Abstract: Metrics of color-difference formula improvement (i.e., standardized residual sum of squares and Pearson product moment correlation) are shown to convey the same information. Furthermore, each metric has two computational forms that assume different linear data models, specifically, with or without an ordinate-intercept. It is essential to choose a computational form that matches the data model. We recommend explicitly declaring whether or not the data have been centered, i.e., by subtracting the mean value from each datum, to match the intercept-free data model. Statistical testing of the metrics assumes independent, normally-distributed randomness of residuals from the data model, and homogeneous variance. Procedures consistent with these assumptions include robust statistical tests, homogenizing data transformations and meta-analysis.

Unitary rotation of pixellated polychromatic images

Alejandro Urzua and Kurt Bernardo Wolf

DOI: 10.1364/JOSAA.462530 Received 03 May 2022; Accepted 15 Jun 2022; Posted 16 Jun 2022  View: PDF

Abstract: Unitary rotations of polychromatic images on finite two-dimensional pixellated screens provide invertibility, group composition, and thus conservation of information. Rotations have been applied on monochromatic image data sets, where we now examine closer the Gibbs-like oscillations that appear due to discrete ‘discontinuities’ of the input images under unitary transformations. Extended to three-color images we examine here the display of color at the pixels where, due to the oscillations, some pixel color values may fall outside their required common numerical range [0,1], between absence and saturation of the red, green, and blue formant color images.

Scintillation of Partially Coherent Light in Time Varying Complex Media

Josselin Garnier and Knut Solna

DOI: 10.1364/JOSAA.453358 Received 10 Jan 2022; Accepted 14 Jun 2022; Posted 15 Jun 2022  View: PDF

Abstract: We present a theory for wave scintillation in the situation with a time-dependent partially coherent source and a time-dependent randomly heterogeneous medium. Our objective is to understand how the scintillation index of the measured intensity depends on the source and medium parameters.We deduce from an asymptotic analysis of the random wave equation a general form of the scintillation index and we evaluate this in various scaling regimes.The scintillation index is a fundamental quantity that is used to analyze and optimize imaging and communication schemes. Our results are useful to quantify the scintillation index under realistic propagation scenarios and to address such optimization challenges.

3D Eye-Tracking Device to Determine Spatiotemporal Point of Regard

Peter Wagner, Arthur Ho, and Juno Kim

DOI: 10.1364/JOSAA.457663 Received 18 Mar 2022; Accepted 10 Jun 2022; Posted 14 Jun 2022  View: PDF

Abstract: This manuscript presents and evaluates a system and method that records spatiotemporal scene information and location of the center of visual attention i.e., spatiotemporal point of regard in ecological environments. A primary research application of the proposed system and method is for enhancing current 2D visual attention models. Current eye-tracking approaches collapse a scene’s depth structures to a 2D image, omitting visual cues that trigger important functions of the human visual system (e.g., accommodation and vergence). We combined head-mounted eye-tracking with a miniature time-of-flight camera to produce a system that could be used to estimate the spatiotemporal location of the point of regard (PoR) – the point of highest visual attention – within 3D scene layouts. Maintaining calibration accuracy is a primary challenge for gaze mapping, hence we measured accuracy repeatedly by matching the PoR to fixated targets arranged within a range of working distances in depth. Accuracy was estimated as the deviation from estimated PoR relative to known locations of scene targets. We found that estimates of 3D PoR had an overall accuracy of approximately 2˚ omnidirectional mean average error (OMAE) with variation over a 1-hour recording maintained within 3.6˚ OMAE. This method can be used to determine accommodation and vergence cues of the human visual system continuously within habitual environments, including everyday applications (e.g., use of hand-held devices).